What if the extra was $399?
What if we could all come to your house and smoke what you are smoking? A regular 1.4 TC is $499, the benchmark the 200-400 set for a built in one is for a substantial premium over that, for instance the Nikon 200-400 f4 costs $6,599, the Canon version with built in TC costs $11,799.
So bearing in mind the current 100-400 costs $1,699, and all MkII's have added at least a $1,000 to MkI prices, now put in a TC and you could easily be looking at $3,500-$4,500. The Nikon 80-400 sells for $2,695 with no TC.
I strongly suspect the only thing people will have to moan about any MkII 100-400 will be the price, oh, and that is is still a push pull design, or that it isn't a push pull design anymore!